1. Field of the Invention
This invention relates to downhole testing apparatus having pumps having pressure limiters and for pumping fluid to inflatable packers, and more particularly, to a pressure limiter which vents to a well annulus and is maintained in an open position until the packers are deflated.
2. Description of the Prior Art
A known method of testing a well formation is to isolate the formation between a pair of inflatable packers with a flow port therebetween adjacent the formation. The packers are inflated by means of a pump in the testing string which pumps well annulus fluid or mud into the packers to place them in sealing engagement with the well bore.
Typically, positive displacement pumps are used. U.S. Pat. No. 4,246,694 to Brandell discloses a rotationally operated pump having a plurality of vertically reciprocating pistons which are driven by a cam structure. A simpler, sleeve-type pump piston is used in the downhole pump of Evans et al., U.S. Pat. No. 3,926,254. Both of these patents are assigned to the assignee of the present invention.
When using these or other pumps for inflating the packers, it is essential that the packers not be overinflated and damaged. One method of limiting the inflation pressure uses a torque limiter. As the work string is rotated to operate the pump and increase the pressure therein, the torque required to operate the pump also increases. When the torque exceeds a predetermined level, the torque limiter is engaged which allows the entire pump to rotate so that it will no longer pump liquid.
Another method utilizes a type of pressure limiter having a spring biased differential piston which, when engaged, engages a set of lugs which allows the bottom of the pump to be held stationary while the top is rotated by the work string so that the pump will operate. When the pump pressure exceeds the spring force acting on the differential piston, the lugs are disengaged which allows the entire pump to rotate so that it no longer pumps. With either of these two methods of limiting inflation pressure, a set of lugs is engaged or disengaged which may be inconsistent in their operation. A large amount of friction results when the lugs are engaged or disengaged, and any side load resulting from a deviated hole also increases the friction. It is therefore desirable to develop a pressure limiter which does not present these frictional difficulties.
U.S. Pat. No. 4,313,495 to Brandell, assigned to the assignee of the present invention, utilizes a clutch which is disengaged when the pump pressure reaches a predetermined level, thus making the pump inoperative. Again, there are frictional limitations with such a system. The present invention which utilizes a sleeve-like relief valve bypasses pump discharge liquid to the well annulus. Continued rotation of the tool string in operation of the pump merely circulates the fluid. No frictionally hindered members are present.
The pressure limiter of White et al. disclosed in U.S. Pat. No. 4,729,430, assigned to the assignee of the present invention, limits packer pressure internally and does not vent fluid therein to the well annulus. In some embodiments of this apparatus, a reciprocating sleeve-like piston increases the pumping chamber volume in response to the displacement of the pump. While this is desirable in some situations, the apparatus is considerably more complex than the pressure limiter of the present invention. The present pressure limiter may also be utilized with any number of previously known positive displacement pumps.